The present invention relates to a conductive ceramic-metal composite body exhibiting positive temperature coefficient (PTC) behavior, which is used to protect electrical and electronic components from damage due to overcurrent conditions.
It is known that ceramic materials which exhibit PTC behavior/characteristics can be used to protect electrical and electronic components against overcurrent conditions, because the resistivity of those materials increases dramatically at specific temperatures. Traditionally, materials like barium titanate have been used in this regard, because the material exhibits an exponential increase in resistivity at its Curie point temperature. However, such materials also have relatively low conductivity at room temperature, thus rendering them unsuitable for many applications, such as consumer electronics.
In view of the drawbacks associated with barium titanate PTC products, the industry has turned to polymer PTC materials for use in electronic components where currents of several tens of milliamperes can be expected. In such polymer materials, conductive particles are dispersed in a polymer matrix to form a conductive path from one side of the matrix to the other. When an overcurrent condition occurs, the polymer matrix is heated above its phase transition temperature (e.g., 120.degree. C. for polyethylene), at which time the volume of the polymer matrix expands and disrupts the conductive path of particles formed therethrough. As a result, the resistivity of the overall material increases substantially and thus prevents the overcurrent condition from damaging downstream electronic components. These materials are attractive in that they have high conductivity and high insulation breakdown strength at room temperature.
One drawback associated with polymer PTC devices is that the trip-point temperature of the device is dictated solely by the phase transition temperature of the polymer used as the matrix. In the case of polyethylene, the phase transition temperature of that polymer material is about 120.degree. C. and thus the trip-point temperature of any PTC device made of polyethylene is limited to about 120.degree. C. Consequently, it is difficult to change the trip-point temperature to account for different overcurrent conditions in different electronic devices.
Another drawback associated with polymer PTC devices is that the PTC effect occurs due to a phase transformation in the matrix material itself, and not in the conductive particles held within the matrix. Accordingly, every time the matrix goes through a phase transformation, the network of conductive particles changes. Consequently, the room temperature resistivity after a trip condition rarely matches the room temperature resistivity before the trip condition. This is undesirable, since circuit designers would like the room temperature resistivity of the PTC device to be the same after every trip condition.
Yet another drawback associated with polymer PTC devices is that, in severe overcurrent conditions, the polymer matrix material can be decomposed to elemental carbon thus leaving a permanent conductive path through the device. Such a permanent conductive path, of course, would allow the overcurrent condition to reach downstream electronic components.
There have been recent reports of ceramic-metal composite PTC devices wherein metal particles, such as bismuth, are disposed in a ceramic matrix to form a conductive path therethrough. Materials such as silica and alumina have been used as the matrix material for these composites, and it is has been demonstrated that these composites show an exponential increase in resistivity at about 280.degree. C. However, the room temperature resistivity is on the order of 1000 .OMEGA..multidot.cm, which is much too high for use in practical applications. Acceptably low room temperature resistivities have been realized only by using semi-insulating materials for the matrix.